Field of the Invention
The present invention relates to a method of producing an aluminum clad member, and more particularly relates to a method which permits advantageous production of an aluminum clad member by superposing an Al skin member formed of aluminum or an aluminum alloy on at least one of opposite major surfaces of an Al core member formed of aluminum or an aluminum alloy, and subjecting the thus obtained stack of the Al core member and the Al skin member to a hot rolling process, so that those members are joined together and integrated into a one-piece body.
Description of Related Art
The aluminum clad member is a sheet or plate member obtained by: superposing the Al skin member on at least one of the opposite major surfaces of the Al core member, and interposing an Al intermediate member formed of aluminum or an aluminum alloy between the Al core member and the Al skin member, as needed depending on an application of the aluminum clad member to be obtained, thereby forming the stack (laminar body) having a multi-layer structure; heating the stack to a predetermined temperature and subjecting the stack to the hot rolling process; and then subjecting the stack to a cold rolling process to reduce the thickness of the stack to a predetermined value. Various methods have been proposed as methods of producing such aluminum clad member (see JP-A-2002-167637, JP-A-2008-264825 and JP-A-2013-220435, for example), and the aluminum clad member has been practically used as a brazing sheet for a heat exchanger for transporting equipment such as an airplane and an automotive vehicle. However, there remain many problems to be solved regarding the conventional methods of producing the aluminum clad member.
Specifically, in the hot clad rolling process described above, the members constituting the stack are heated to a high temperature and subjected to a high pressure, whereby those members are joined together at their interface. This joining is generally called “pressure welding”. In this respect, it is noted that surfaces of the members constituting the stack such as the Al core member and the Al skin member are covered by oxide films. Accordingly, in order to join those members together, it is necessary to destruct the oxide films. However, the oxide films cannot be destructed by merely applying the high pressure to those members in a direction perpendicular to their joint surfaces, in the hot clad rolling process, resulting in difficulty in joining those members together. Therefore, it is a general knowledge among those skilled in the art that it is necessary to destruct the oxide films by forcing the members of the stack to slide at their interface.
In fact, where a clad ratio of the skin member is excessively high, it is difficult to cause sliding of the core member and the skin member at their interface, resulting in the difficulty in joining them together. On the other hand, where the clad ratio of the skin member is excessively low, a temperature of the skin member decreases before the hot clad rolling process, so that it is difficult to destruct the oxide films. Further, the excessively low clad ratio of the skin member gives rise to a problem that rolling defects tend to be caused by tearing of the skin member during the hot rolling process, for example. In view of the above-described problems regarding the production of the clad member, the clad ratio of the aluminum clad member is generally limited to a value within a range of 5-25%. Further, it is considered difficult to produce the aluminum clad member by using 5000 series, 6000 series and 7000 series Al alloys according to JIS, as materials of the core member and the skin member, since those Al alloys contain a large amount of magnesium (Mg), and have stiff oxide films on their surfaces in the presence of a Mg oxide (MgO) as well as an aluminum oxide (Al2O3) formed on their surfaces.
By the way, in order to destruct the oxide films, it is generally considered effective to cause the sliding of the Al core member and the Al skin member at their interface during the clad rolling process, as described above. However, the sliding of those members results in an adverse effect on a clad ratio distribution. Namely, the sliding of the Al core member and the Al skin member at their interface is caused by a difference between an amount of elongation of the Al core member and an amount of elongation of the Al skin member during the rolling process. In this respect, it is noted that in the rolling process performed to produce the clad member, a portion of the stack closer to rolls is easier to be elongated in a rolling direction. Accordingly, where the strength of the skin member is substantially equal to or lower than that of the core member, the amount of elongation of the skin member is considerably larger than that of the core member. On the other hand, where the strength of the core member is lower than that of the skin member, the amount of elongation of the core member is sometimes considerably larger than that of the skin member. In either of the above-described cases, the Al core member and the Al skin member are elongated by the same amount after those members are sufficiently joined together at their interface. However, in an early stage of the rolling process, joining of the core member and the skin member at their interface is insufficient, and the difference between the amounts of their elongation is affected by many factors such as the materials of the core member and the skin member, the clad ratio, a rolling temperature and a change in a rate of reduction (rolling reduction rate). Accordingly, it is difficult to precisely anticipate the difference between the amounts of elongation of the core member and the skin member before those members are sufficiently joined together at their interface, resulting in reduction of accuracy of the clad ratio.
On the other hand, the sliding of the core member and the skin member at their interface is remarkable in front and rear end portions of the stack as seen in the rolling direction, and in right and left end portions of the stack as seen in its width direction, in which end portions, resistance to the sliding is lower than that in the central portion of the stack. Accordingly, the clad ratios in the front and rear end portions of the stack as seen in the rolling direction and the right and left end portions of the stack as seen in its width direction differ from the clad ratio in the central portion of the stack. For instance, where the skin member is easier to be elongated than the core member, the clad ratio decreases in the front and rear end portions and the right and left end portions of the stack. Therefore, after the clad member is obtained by the hot rolling process, its front and rear end portions and right and left end portions having the clad ratio outside the tolerable range are generally cut off, resulting in considerable reduction of an yield of the clad member and a considerable increase of its cost.
The difference of the clad ratio among different portions of the aluminum clad member, in other words, a variation in the clad ratio distribution is presumably generated for reasons described below. In production of the aluminum clad member, the hot clad rolling process is generally performed at a rolling reduction rate as low as not higher than several %, in the early stage, for purposes of: 1) elongating the Al skin member (flattening the Al skin member where the skin member has a higher strength than the core member) so that the Al skin member is brought into even surface contact with the Al core member or the Al intermediate member; and 2) surface joining the Al core member and the Al skin member together. In this respect, it is noted that the surface joining of the Al core member and the Al skin member is not completed in this stage. Particularly in the case where the Al skin member is obtained by a hot rolling process and used without being subjected to any other process after the hot rolling process, and where the Al skin member has a low degree of flatness, first several passes of the hot clad rolling process for producing the clad member are performed for elongating or flattening the Al skin member, and the Al skin member and the Al core member are not joined together in those first several passes. Also, even in some cases where a machining operation is performed with respect to a surface of the Al skin member to be joined to the Al core member or the Al intermediate member, in order to improve the flatness of the above-described surface, only the Al skin member or the Al core member is substantially elongated in the first several passes of the hot clad rolling process. At this time, in the case where only the Al skin member is elongated, the clad ratio is initially reduced over the entire area of the clad member, and an amount of reduction of the clad ratio is more remarkable in the end portions of the clad member as described above, in which end portions, resistance to elongation of the Al skin member is relatively low.
In the subsequent stage wherein the joining of the Al core member and the Al skin member at their interface has progressed to some extent, the rolling reduction rate is slightly increased to elongate the Al core member and the Al skin member by a large amount with a strong rolling force, and to destruct the oxide films by generating a strong friction force at the joint interface between the Al core member and the Al skin member, so that those members are completely joined together. However, the joining of the Al core member and the Al skin member is not completed concurrently and uniformly over the entire area of the joint interface. In regions of the joint interface where the Al core member and the Al skin member are not sufficiently joined together, if the strength of the Al skin member is lower than that of the Al core member, reduction of the thickness of the Al skin member caused by its elongation preferentially proceeds as compared with reduction of the thickness of the Al core member, resulting in partial reduction of the clad ratio. This partial reduction of the clad ratio also takes place in a portion of the clad member other than its front and rear end portions and right and left end portions, resulting in unevenness and a variation of the clad ratio in the portion of the clad member, which portion will not be cut off and will be obtained as the end product.
As described above, the clad ratio of the clad member as a whole does not have a sufficiently high degree of accuracy, due to the difference in the strength of the Al core member and the Al skin member, for example, and the clad ratio considerably deviates from the target value in the front and rear end portions and the right and left end portions of the clad member, which end portions will be cut off. Further, the clad ratio is uneven and has the variation even in the portion of the clad member which will not be cut off and will be obtained as the end product.